For predicting the transient fresh-salt water interface in confined coastal aquifers, an equation has been developed to estimate the advance of interface. The equation depends on; the densities of fresh and salt water, the hydraulic properties of the aquifer and the rate of flow of groundwater in the fresh-water region. The developed equation is applied on a confined aquifer example to compute the average advance of interface. The estimated analytical value is found to have the same order of magnitude and nearly equal to the value estimated using the Ghyben-Herzberg relation, where the deviation is about 0.02. The agreement is considered satisfactory, since the developed equation considers the hydraulic properties and the actual hydrological conditions of the aquifer. However, the developed analytical solution seems to be successful procedure to be applied for predicting the transient interface in confined coastal aquifers.